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Comparative genomic analysis of three geographical isolates from China reveals high genetic stability of Plutella xylostella granulovirus
Comparative genomic analysis of three geographical isolates from China reveals high genetic stability of Plutella xylostella granulovirus

Man-Li Zhang, Ling-Qian Wang, Yong Qi, Yi Wu, Dong-Hui Zhou, Lu-Lin Li

Competing Interests: The authors have declared that no competing interests exist.

https://doi.org/10.1371/journal.pone.0243143, Volume: 16, Issue: 1, Pages: 1-23
Article Type: research-article Article History
Publisher: Public Library of Science
Abstract

In this study, the genomes of three Plutella xylostella granulovirus (PlxyGV) isolates, PlxyGV-W and PlxyGV-Wn from near Wuhan and PlxyGV-B from near Beijing, China were completely sequenced and comparatively analyzed to investigate genetic stability and diversity of PlxyGV. PlxyGV-W, PlxyGV-B and PlxyGV-Wn consist of 100,941bp, 100,972bp and 100,999bp in length with G + C compositions of 40.71–40.73%, respectively, and share nucleotide sequence identities of 99.5–99.8%. The three individual isolates contain 118 putative protein-encoding ORFs in common. PlxyGV-W, PlxyGV-B and PlxyGV-Wn have ten, nineteen and six nonsynonymous intra isolate nucleotide polymorphisms (NPs) in six, fourteen and five ORFs, respectively, including homologs of five DNA replication/late expression factors and two per os infectivity factors. There are seventeen nonsynonymous inter isolate NPs in seven ORFs between PlxyGV-W and PlxyGV-B, seventy three nonsynonymous NPs in forty seven ORFs between PlxyGV-W and PlxyGV-Wn, seventy seven nonsynonymous NPs in forty six ORFs between PlxyGV-B and PlxyGV-Wn. Alignment of the genome sequences of nine PlxyGV isolates sequenced up to date shows that the sequence homogeneity between the genomes are over 99.4%, with the exception of the genome of PlxyGV-SA from South Africa, which shares a sequence identity of 98.6–98.7% with the other ones. No events of gene gain/loss or translocations were observed. These results suggest that PlxyGV genome is fairly stable in nature. In addition, the transcription start sites and polyadenylation sites of thirteen PlxyGV-specific ORFs, conserved in all PlxyGV isolates, were identified by RACE analysis using mRNAs purified from larvae infected by PlxyGV-Wn, proving the PlxyGV-specific ORFs are all genuine genes.

Zhang,Wang,Qi,Wu,Zhou,Li,and Zhou: Comparative genomic analysis of three geographical isolates from China reveals high genetic stability of Plutella xylostella granulovirus

Introduction

Baculoviruses have long been explored as biological control agents of agricultural and forest pests attributing to their pathogenicity highly specific for insects, mainly Lepidoptera, Hymenoptera and Diptera [1]. Genotypic variation in baculovirus populations have been widely detected between isolates from different geographical regions and within virus isolates by their genomic restriction endonuclease (REN) profiles in 1970s-1990s [2]. Differences in phenotypes were also revealed between different isolates and between genotypes derived from the single isolates by in vitro or in vivo techniques [2, 3]. More recently, hundreds of baculoviruses genomes have been completely sequenced, including the genomes of multiple different geographical isolates of some virus species. Nucleotide polymorphisms (NPs) have been documented between the different virus isolates and within the same isolate [3–8]. Comparative analysis of these genome sequences may make it possible to determine the genetic basis for phenotypic variations in populations of the same viral species. This may facilitate the improvement of baculovirus pesticides by mixing different virus genotypes. The codling moth, Cydia pomonella, was reported to be resistant to a Cydia pomonella granulovirus isolate CpGV-M. However, the resistance can be overcome by several other CpGV isolates. Whole-genome sequencing and phylogenetic analyses of these geographic CpGV variants revealed that the resistance is the consequence of a mutation in viral gene pe38 [9, 10].

Plutella xylostella granulovirus (PlxyGV) belongs to the genus Betabaculoviruses. It is pathogenic for the diamondback moth, Plutella xylostella, a major destructive pest of cruciferous crops worldwide [11]. The virus has been isolated in several countries including Japan, China, India, Kenya, and South Africa [12–17]. In China, PlxyGV was first isolated, in Wuhan, and studied in 1970s [18]. Subsequently, it was isolated in other districts [19, 20]. Some strains of the diamondback moth have developed resistance to chemical pesticides and also become resistant to the bacterial insecticide Bacillus thuringiensis that have been used for its control [21, 22]. As an alternative, PlxyGV has been tested for the control of the pest [16, 19, 20, 23]. A registered PlxyGV biopesticide has been commercialized and used in large scale for the control of diamondback moth in China and Malaysia since 2008 [24–29]. Laboratory experiments have also been done to characterize PlxyGV morphology, histopathology, in vitro replication in cell culture, and molecular biology [17, 30–36].

PlxyGV has a single circular double-stranded DNA genome. The complete genome sequence of a PlxyGV isolate (PlxyGV-K1) from Japan was first reported in 2000 to consist of 100,999 bp and encode 120 putative protein-coding open reading frames (ORFs) [37]. Subsequently, the complete genome sequences of five additional isolates from mainland China (PlxyGV-C and PlxyGV-T), Taiwan (PlxyGV-K), Malaysia (PlxyGV-M), and South Africa (PlxyGV-SA) were published in 2016 [38, 39].

In this study, the genomes of three PlxyGV isolates named PlxyGV-W, PlxyGV-B and PlxyGV-Wn were completely sequenced. Intra isolate NPs and inter isolate NPs in the genomes were detected; And their insecticidal activity to the larvae of diamondback moth were also evaluated. In order to investigate genetic diversity and stability of PlxyGV, the genome sequences of these PlxyGV isolates and six previously reported PlxyGV genome sequences were comparatively analyzed, in nucleotide sequence variations, non-synonymous sequence polymorphisms, gene content and phylogeny.

Materials and methods

Virus and insects

The PlxyGV-W and PlxyGV-Wn were isolated near Wuhan in 1979 and 2018 from diseased P. xylostella larvae in cabbage fields, respectively [18]. PlxyGV-B is from a commercialized biopesticide, that was originally isolated in Beijing in 1980s. The isolates were propagated by feeding an artificial diet contaminated with the virus occlusion bodies (OBs) to third instar laboratory reared Diamondback moth larvae.

Purification of OBs and extraction of viral DNA were carried out as described by Hashimoto et al. [40], with modifications. Approximately100 infected larvae were homogenized with a blender. The worm-tissue fragments in the homogenate were removed by differential centrifugation at 750 RPM for 15min and 8,500 RPM for 25min, and repeated twice. The pellet was suspended in 3‒4 ml of ddH2O. The suspension was layered onto a 30, 40, 50, and 60% (wt/vol) discontinuous sucrose gradient and centrifuged at 4,000 rpm for 1h. The OB fraction was collected and washed twice by suspension in H2O and centrifugation. The OBs were dissolved in equal volume of alkaline solution (100 mmol/L NaCl, 100 mmol/L Na2CO3, 5 mmol/L EDTA) and incubated at room temperature with stirring, then mixed with equal volume of protein digestive solution [1 mmol/L EDTA, 1% SDS (w/v), 10 mmol/L Tris-HCl (PH 7.4), 0.5 mmol/L NaCl, 0.2 mg/L protease K], incubated in a bath at 58°C overnight. Viral DNA was extracted twice with an equal volume of phenol-chloroform and precipitated by mixing with ethanol centrifugation, then dissolved in TE buffer and stored at 4°C.

Genome sequencing

The genomes of the three PlxyGV isolates were sequenced by using Illumina Hisen X ten system. Sequence assembly were done by using SOAP denovo (Version 2.04) software (BGI), and using the first published genome sequence of the PlxyGV-K1 isolated from Japan (NC_002593) as a reference. PCR was performed to synthesize DNA fragments bridging the gaps between contigs by using the genomic DNAs of individual PlxyGV isolates as templates. PCR products were sequenced from both ends. The sequences were assembled with the initial contigs into a single, circular contig. Sequences were analyzed with Lasergene programs (DNASTAR). Homology searches were carried out with GenBank/EMBL, SWISSPROT and PIR databases by using the BLAST algorithm. Multiple sequence alignments were performed by using CLUSTAL W. The PlxyGV genome sequence accession numbers are MN099284 for PlxyGV-W, MN099285 for PlxyGV-B and MN099286 for PlxyGV-Wn.

RNA purification and RACE analysis of PlxyGV-specific genes

P. xylostella larvae in third instar were infected with PlxyGV-Wn by feeding with viral OBs-contaminated diet and collected at 12 h, 24 h, 48 h, 72 h and 96 h post infection. 25 infected larvae (five larva from each time point) were immersed and homogenized in 1,000 μl of Trizol and incubated on ice for 10 min, then centrifuged at 11,400 rpm and 4°C for 10 min. The supernatant was mixed with 200 ml of chloroform with shaking for 15 s, and incubated on ice for 15 min, then centrifuged at 11,400 rpm and 4°C for 15 min. 400 μl of the upper phase was taken and mixed with 500 μl of isopropyl alcohol, incubated at room temperature, then centrifuged at 11,400 rpm for 10 min. The pellete was rinsed with 200 μl of 75% ethanol in DEPC water by centrifugation at 11,400 rpm for 5 min, air dried, then dissolved in x μl of DEPC water. 8 μl of RNA sample was mixed with 1 μl of DNA digestion buffer and 1 μl of DNase I, and incubated at 37°C for 30 min. Then 2 μl of 50 mM EDTA was added to inactivated DNase I by incubation at 65°C for 10 min.

First-strand cDNAs synthesis and RACE were done by using SMARTer® RACE 5′/3′Kit following manufacturer’s instruction. For first-strand cDNAs synthesis, mixed 4.0 μl of 5x first-strand buffer, 0.5 μl DTT (100mM) and 1.0 μl dNTPs (20 mM) to make reaction buffer mix, at first. Send, mixed 1.0 ‒10 μl of RNA, 1.0μl 5’-CDS primer A (or 3’-CDS primer A) and 0 ‒10μl DEPC water, incubated the mixtures at 72°C for 3 min, cooled down at 42°Cfor 2 min, then spun at 14,000g for 10s, to make denatured RNAs. To the RNA sample for 5’RACE cDNA synthesis, add 1 μl of SMART II A Oligonucleotide. Then, mixed 5.5 μl of 5’RACE or 3’RACE cDNA synthesis reaction buffer mix, 0.5 μl RNase inhibitor (40 U/μl) and 2.0 μl SMARTScribe Reverse Transcripase (100 U), to make 5’RACE or 3’RACE cDNA synthesis master mix. Finally, combined 8 μl of the 5’RACE or 3’RACE cDNA synthesis master mix with the denature RNA sample, incubated at 42°C for 90 min, then heated at 72°C for 10 min. Diluted the sample by adding 10 μl (if started with ≤ 200 ng of total RNA) or 100 μl (if started with≧200 ng of total RNA) of Tricine-EDTA buffer to make 5’ RACE-ready cDNA and 3’-RACE-ready cDNA. To perform RACE, mixed 25 μl of 2x seqAmp buffer, 2.5 μl of 5’(3’) RACE-ready cDNA, 5 μl of 10x universal primer A mixture, 1 μl of 5’(3’) RACE primer, 1 μl of SeqAmp DNA polymerase and 15.5 μl of PCR-grade water, and ran PCR as below: first step: 94°C,4 min. Second: 94°C, 30s; 68°C, 30s; 72°C, 3min; 25 cycles. Third: 72°C, 7 min.

Bioassays

Bioassays on the infectivity of PlxyGV isolates were performed as previously described [41]. To determine the median lethal concentration (LC50), virus suspensions in concentrations of 0, 1×107, 2×107, 5×107, and 1×108 OBs/ml were prepared respectively, by suspending the virus OBs in 4% sucrose in double-distilled water containing 0.05% food blue dye. Quantification of PlxyGV was performed by using a qPCR method. The virus suspensions were used to feed newly molted third-instar P. xylotella larvae that had been starved for twelve hours. The larvae that had swallowed the virus suspension were picked and transferred into the wells of twelve-well plates and feed with fresh artificial diet for the duration of the bioassay. Mortality was recorded daily after infection until larvae died or pupated. Forty eight larvae per concentration were used in the infection experiments and the experiments were repeated in triplicate. The LC50 values were determined by the probit analysis calculated compared with a relative median potency method. To determine median lethal time (ST50), newly molted third-instar P. xylotella larvae were oral infected in the same way as above, using virus suspensions of 1×109 OBs/ml. Mortality was recorded every 6 h after infection until all larvae died or pupated. The ST50 values were calculated with the Kaplan–Meier estimator and compared by the log-rank test.

Results

Genome sequences of PlxyGV-W, PlxyGV-B and PlxyGV-Wn

The genomes of PlxyGV-W, PlxyGV-B and PlxyGV-Wn are 100,941 bp, 100,972 bp and 100,999 bp in length with a G + C compositions of 40.73%, 40.71% and 40.71%, respectively. A complete sequence alignment showed a sequence identity of 99.8% between PlxyGV-W and PlxyGV-B, 99.6% between PlxyGV-W and PlxyGV-Wn, and 99.5% between PlxyGV-B and PlxyGV-Wn, respectively. The gene contents, genome organization and variations of the three isolates are demonstrated by Fig 1. It is shown that all the three PlxyGV isolates contains 118 ORFs in common, being 150 bp or longer, starting with an ATG and having minimal overlap with adjacent ORFs or homologous repeat regions (hrs), respectively. All the homologous ORFs and hrs are completely collinear in organization in the three isolates.

Comparison of genome structure between PlxyGV-W, PlxyGV-B, and PlxyGV-Wn.
Fig 1

Comparison of genome structure between PlxyGV-W, PlxyGV-B, and PlxyGV-Wn.

The figure depicts a schematic representation of the PlxyGV genomes with map positions of the 118 ORFs represented by arrows indicating transcriptional direction and relative size. Numbers above arrows represent the number of each ORF. Blue and orange vertical lines represent the locations of synonymous and nonsynonymous nucleotide polymorphisms respectively. Light blue and red vertical lines represent the locations of synonymous and nonsynonymous point mutations in the PlxyGV-B and PlxyGV, compared to PlxyGV-W. Raised lines represent insertions and the lowered lines represent deletions. Turning lines represent frameshift mutations. hr sequences and their positions on the genome are indicated by light grey boxes. Empty arrows represent ORFs with nonsynonymous mutations relative to PlxyGV-W.

Intra-isolate NPs are detected in all the three virus isolates. In PlxyGV-W genome, thirty two single nucleotide polymorphisms (SNPs) and four NPs involving two or more nucleotide alterations are identified. Although the majority of the NPs locate in ORFs, only eight SNPs encode amino acid alterations, occurring in ORF26 (f), ORF32 (lef2), ORF61 (dbp), ORF99 (lef9), ORF104 (fgf), ORF109 (lef8) and ORF113, respectively, and there is a NP with a insertion/deletion (InDel) of twelve nucleotides after AA68 and a deletion of a single nucleotide causing frame shift after AA101, in ORF73 (ac91) (Table 1).

Table 1
Intra-isolate nucleotide polymorphisms in PlxyGV-W, PlxyGV-B and PlxyGV-Wn.
ORFName/DirPlxyGV-WPlxyGV-BPlxyGV-Wn
ntaantaantaa
C152CG
1→A225G
G289AG289A
G309AG309A
210/←A391GA391G
C486T
C511AM33I
3→C681T
4Gran/→G1031C
A1064G
T1154G
A1175G
C1274T
T1280C
C1391T
A1520G
5←G1865TG1865T
7Pif1/←T3443C
A3458G
10ie1/←T6368C
11ac146/→T6807C
1449k/←G8105AG8105A
16pif5/←T10608GK32T
20←G12756GAGAAGGGCATCTTCGAGGP89PRRCPSP
T13017A
T14324GG14306T
T14495AT14477A
C14505TT14487CC14471T
T14530A
G14662GTG14644GT
22→G15085AG15070A
G15839A
24→T16717C
T17377C
25→C17775T
T18084C
A18519G
C18690G
26f/→A19111C
T20360GL494RT20342GL494R
G20349T
28←G21056A
29pif3/→A21832G
30odv-e66/←G22527A
C22992TC22974T
A23208G
32lef2/→G24631AD69NG24613AD69N
C24927AN173K
C27728T
34←G28191A
G28270A
35Mmp/←T28615C
A28659G
T28702C
A29080G
C29398T
A29500C
T29721CM10V
36p13/→C30478T
39→A32122G
40ac106/107/←C33552TC33574T
G33696AG33718A
41pif7/←C33910TC33932T
T35671G
T39530G
hr2G39699T
CT40366C
C41711T
C41929G
A41932G
50p10/←C42272T
51p47/→T43504C
52ac38/→T43829C
C44078TR139K
53p24/→A44488TT44512A
A44692TA44716T
54ac13/←G45080A
G45213T
55lef1/←G45666A
A45729T
57←C46734T
G47492A
61Dbp/←A47648GA47672G
T47735CT47759C
T48028CI82VC48052TV82I
65→G50238C
A50279C
66ac101/→A50703GA50729G
C51015A
67p6.9/→G51747A
68ac145/→A51839ATTG
G51857A
69lef5/←T52117C
G52152T
A52296G
A52371G
7038k/→T52976C
C53271TC53295T
A53545GK321E
71pif4/←A53664GA53688G
72hel1/→G54288A
C54366T
A54513G
A54675G
G55573A
C55711TC55735T
A56019GA56043G
C56208TC56232T
G57350AK1110R
73ac91/→A57477C
T57488CT28I
C57553T
A57592TL54F
TCCACCGCCGACC57552-TTPPPT68-72TT57575TCCACCGCCGACCT68TPPPTT57602CT69P
A57684AC101fram shiftAC57697A101fram shift
G57755A
A57756C
A57758T
A57761T
G57855AG57869A
74odv-e25/←A58307T
76ac92/→T59321C
77←T59853C
80odv-ec27/→A62986G
G63984A
hr3G64044C
G64045T
G64052A
C64182T
C64183T
G64194A
84pif8/←T67695CV109T
85tlp20/→G68166A
87gp41/→A69375T
T69454G
89vlf1/→C70312T
90←C71254T
93Dnapol/←G74855A
G77305A
hr4TTGTTAAAATAAATAAACATTC77980T
T78637A
97→A80551G
C80554A
98Iap/→A80957G
99lef9/→G81575AR50QG81606AQ50R
T82198GS258AT82229GA258S
T82326C
G82434AG82465A
101dna-lig/←A84127G
T84914CT84941C
104Fgf/←C85724TI318V
A86038GF203LA86069GF203L
105←G86805A
G86891AG86922A
A86996G
C87801T
106alk-exo/→A88162G
C88804A
107hel2/→C89169T
G89201A
G89858A
G89527A
G90072A
109lef8/←T92293CF655LT92324CF655L
A93448G
112ac53/→A95747GF79L
A95854GV24AA95885GV24A
115vp1054/→C96394T
C96644T
T97116C
A97526G
120me53/→C100273T
A100611G
A100788G
T34CC34T

*Nucleotide polymorphisms (nt) and amino acid changes (aa) encoded are presented. The numbers between letters indicate the positions in the genomes or in ORFs.

In the PlxyGV-B genome, there are 105 SNPs and six NPs involving multiple nucleotide alterations. The majority of the NPs identified in PlxyGV-W are also found in PlxyGV-B. Nineteen NPs causing amino acid changes in thirteen ORFs of PlxyGV-B genomes including ORF2 (p10), ORF20, f, lef2, ORF35 (mmp), dbp, ORF68 (ac145), ORF72 (hel-1), ORF73 (ac91), ORF84 (pif8), lef9, fgf and lef8 and ORF112 (ac53). lef2, lef9 and fgf contain two, and ORF73 contains three nonsynonymous NPs (Table 1). Similar to PlxyGV-W, there is an identical NP with an InDel of twelve nucleotides and an InDel of a single nucleotide causing frame shift in ORF73. The difference is that the sequencing reads missing the twelve nucleotides are more than the ones with the twelve nucleotides, in PlxyGV-W ORF73. In contrast, the sequencing reads missing the twelve nucleotides are much less than the ones with the twelve nucleotides, in PlxyGV-B ORF73. In PlxyGV-B ORF20, there is an NP involving an InDel of eighteen nucleotides. PlxyGV-Wn genome contains sixty nine SNP sites. Only six SNPs in five ORFs induce amino acid alterations, including ORF16 (pif5), ORF52 (ac38), ORF70 (38k), ORF73 (2 SNPs) and ORF113 (Table 1). Most SNPs in PlxyGV-Wn genome are different from the ones in PlxyGV-W and PlxyGV-B genomes. Majority of the SNPs in all the three PlxyGV isolates are nucleotide transitions.

Comparison of the genomes of PlxyGV-W and PlxyGV-B

PlxyGV-B genome is 31 bp longer than PlxyGV-W genome. The difference is mainly in the hr regions. The hr1, hr2, hr3, and hr4 of PlxyGV-B are 40 bp, 2 bp, 3 bp and 15 bp longer than those of PlxyGV-W, respectively. The sizes of all putative protein-coding ORFs of PlxyGV-B are same as the ones of PlxyGV-W except ORF20 and ORF73 (Table 2). Relative to PlxyGV-W ORF20, PlxyGV-B ORF20 has six amino acids deleted after AA119. And there are twelve additional amino acid variations between these two ORF20 homologs. As mentioned above, both PlxyGV-W and PlxyGV-B ORF73 homologs have two NPs at AA68 and AA101 (PlxyGV-W)/94(PlxyGV-B) sites. In PlxyGV-W ORF73, most sequencing reads have the extra twelve nucleotides encoding “TPPP” after AA68, and a small part of sequencing reads miss the twelve nucleotides. In contrast, most sequencing reads miss the twelve nucleotides and small parts of sequencing reads have the ones in PlxyGV-B ORF73. At the AA101/97 site, most sequencing reads contains an “A”, small part of sequencing reads contains “AC” that makes a frame shift. In contrast, most sequencing reads contains “AC” and small part of reads contain “A”, in PlxyGV-B ORF73. There are seven additional nonsynonymous variations in six ORFs between PlxyGV-B and PlxyGV-W (Fig 1 and Table 2).

Table 2
Nonsynonymous mutations in PlxyGV-W, PlxyGV-B and PlxyGV-Wn.
ORFNameSize (aa)PlxyGV-WPlxyGV-BPlxyGV-Wn
2P108349S49S49N@
7PIF1536385R385R385K
817518V, 110N, 137A18V, 110N, 137A18I, 110T, 137T
98062I62I62M
10IE1393129D, 320G129D, 320G129G, 320E
11AC146180116V116V116A
15EXON021645I, 120L45I, 120L45M, 120S
16PIF535132T32T32K
18313102T, 185V102T, 185V102P, 185A
20229/223/22952C, 57W, 66S, 69L, 79LI, 88RRRR, 94A, 114RCPSPR52N, 57E, 66C, 69A, 79SL, 88PPSC, 94P, 114∧652C, 57W, 66S, 69L, 79LI, 88RRRR, 94A, 114RCPSPR
25431134E134E134K
26F553494L494L494R
30ODV-E66682205F205F205Y
32LEF227069D, 173N69D, 173N/K69N, 173K
35MMP40210M10M10V
37PIF2368199F, 202S199F, 202S199L, 202G
38528181K, 206Q181K, 206Q181E, 206K
43AC109414122V, 381S122V, 381S122A, 381T
49PIF0627286A286A286T
56FGF2216S, 149D6S, 149D6Y, 149E
61DBP26382I82V82V
65152115E115E115K
69LEF5247212R212R212K
7038K340321E321E321K
72HEL1112497I, 531N, 665N, 1110R97I, 531N, 665N, 1110K/R97N, 531S, 665D, 1110R
73AC91153/159/15528I, 54F, 69TPPP/∧, 103-153fs/n28T/I, 54F, 69∧4/TPPP, 103∨PPPPPPPP28T, 54L/F, 69T/PPPP, 103∧8
75AC9315624D24D24E
78LEF443232D, 91I32D, 91I32E, 91T
8110743W43W43G
82340232Y232Y232C
836932R32R32K
84PIF853366H, 109I, 481S66H, 109V 481S66Y, 109V, 481A
85TLP2013971C71C71S
89VLF134675I,89N,75I,89N,75V, 89K
93DNAPOL978345K345K345R
94AC66651291A291A291S
95LEF3337179R,180I179R,180I179K, 180M,
96PIF6128123R,123R,123H
99LEF949450R, 258S50Q, 258A50Q, 258A
101DNA-LIG523383V383V383I
10363/63/6663∨LKK
104FGF396203F,318V203F, 318I203L, 318I
107HEL243612S12S12G
109LEF8838479I, 655F479I, 655F479M, 655L
112UBI-LIG13761I,117T61I,117T61M, 117I
11319824V,79L24V,79L24A, 79F
118EGT42969Q, 394N69Q, 394N69K, 394D
120ME53260134R134R134G

@ The numbers in front the letters indicate the positions of mutations; ∨ indicates insertion; ∧ indicates deletion; the numbers following ∧ indicate number of codons missing; fs, frameshift; Single isolate specific variations are printed in grey color.

Comparison of PlxyGV-Wn genome with the genomes of PlxyGV-W and PlxyGV-B

PlxyGV-Wn genome is 58 bp and 27 bp longer than the genomes of PlxyGV-W and PlxyGV-B, respectively. Similarly, the differences are also mainly due to the differences in the length of the hrs. There are totally 486 nucleotide substitutions between the genomes of PlxyGV-Wn and PlxyGV-W, including seventy three nonsynonymous point mutations in forty seven ORFs (Table 1 and Fig 1). Twelve positions with nonsynonymous point mutation have polymorphisms in either both or one virus isolate. In addition, there are 262 nucleotide InDels. Relative to PlxyGV-Wn ORF73, PlxyGV-W ORF73 has an insertion of 23 nt after AA101, resulting in a frame shift at C-terminal. However, PlxyGV-W ORF73 has a polymorphism at this part as described above. A single nucleotide substitution at the 3’-end of ORF103 has the stop codon in PlxyGV-W changed to leucine codon and adds additional two codons to the C-terminal in PlxyGV-Wn. The sizes of the other ORFs are identical between PlxyGV-Wn and PlxyGV-W.

There are 469 nucleotide substitutions between the genomes of PlxyGV-Wn and PlxyGV-B. Relative to PlxyGV-B, PlxyGV-Wn genome has 144 nt insertions and 117 nt deletions. These nucleotide mutations encode seventy seven nonsynonymous changes in forty six ORFs. Majority of nonsynonymous variations between PlxyGV-Wn and PlxyGV-B are same as the ones between PlxyGV-Wn and PlxyGV-W with a few exceptions (Table 2 and Fig 1). Relative to PlxyGV-B, PlxyGV-Wn ORF73 has four codons inserted after AA68 and a cluster of eight proline codons deleted after AA95. Unlike PlxyGV-B ORF20 that contains five nonsynonymous mutations relative to PlxyGV-Wn ORF20, PlxyGV-W ORF20 encodes the same amino acid sequences as PlxyGV-Wn ORF20. ORF61 and ORF99 contain one and two nonsynonymous variations between PlxyGV-Wn and PlxyGV-W whereas there is no difference in these two ORFs between PlxyGV-Wn and PlxyGV-B.

PlxyGV-W and PlxyGV-B demonstrate higher insecticidal activity than PlxyGV-Wn for P. xylotella larvae

The infectivity of PlxyGV-W, PlxyGV-B and PlxyGV-Wn were tested for newly molted third-instar P. xylostella larvae by feeding the larvae with viral OBs and determining LC50 and ST50 in bioassays. As shown in Table 3, the LC50 of PlxyGV-Wn is about two times of the ones of the other two virus isolates while there is no significant difference between PlxyGV-W and PlxyGV-B. No significant difference is detected in ST50 between all the three isolates at a concentration of 1×109 OBs/ml (Table 4).

Table 3
Concentration-mortality of PlxyGV-W, PlxyGV-B and PlxyGV-Wn for Plutella xylostella larve.
VirusLC50 (95%CI) (×107OB/mL)Relative median potency (95%CI)
PlxyGV-WPlxyGV-B
Test1PlxyGV-W1.695 (1.171–2.335)--
PlxyGV-B1.570 (1.080–2.162)0. 926 (0.583–1.457)-
PlxyGV-Wn3.131 (2.284–4.291)1.847 (1. 183–3.085)1.995 (1. 275–3.372)
Test2PlxyGV-W2.300 (1.318–4.155)--
PlxyGV-B3.375 (1.352–4.199)1.033 (0.472–2.374)-
PlxyGV-Wn4.516 (3.002–7.292)1.964 (0.979–5.903)1.901 (0.964–5.350)
Test3PlxyGV-W2.282 (1.393–3.840)--
PlxyGV-B2.371 (1.441–3.921)1.039 (0.520–2.168)-
PlxyGV-Wn5.038 (3.480–7.865)2.208 (1.148–6.195)2.125 (1.128–5.594)
Table 4
Time-mortality of PlxyGV-W, PlxyGV-B and PlxyGV-Wn for Plutella xylostella larve.
VirusLT50±SEM (95%CI)(h)Log Rank(Mantel-Cox)
PlxyGV-WPlxyGV-B
χ2Pχ2P
Test 1PlxyGV-W101±3.758 (93.634–108.366)----
PlxyGV-B113±2.761 (107.589–118.411)0.0940.759--
PlxyGV-Wn119±3.779 (111.593–126.407)4.0930.0433.5980.058
Test 2PlxyGV-W117±3.012 (111.097–122.903)----
PlxyGV-B123±2.207 (118.675–127.325)3.5540.059--
PlxyGV-Wn123±2.207 (118.675–127.325)1.7960.1800.0990.753
Test 3PlxyGV-W119±1.685 (115.65–122.35)----
PlxyGV-B125±5.271 (117.016–132.984)1.0140.314--
PlxyGV-Wn125±6.854 (117.016–132.984)2.7020.1001.0980.295

Comparison of the genome sequences of nine PlxyGV isolates

To investigate diversity of PlxyGV isolates from different area, the genome sequences of PlxyGV-W, PlxyGV-B, PlxyGV-Wn are compared with six additional complete PlxyGV genome sequences available in data bases: PlxyGV-C (KU529791.1, 100,980 bp), PlxyGV-K (KU529794.1, 100,978 bp), PlxyGV-T (KU529792.1, 101,004 bp), PlxyGV-M (KU529793.1, 100,986 bp), PlxyGV-SA (KU666537.1, 100,941 bp), and PlxyGV-K1 (NC_002593.1, 100,999 bp). Complete genome sequence alignments show that sequence identities between the genomes of all the PlxyGV isolates, except PlxyGV-SA, are over 99.4% (Fig 2). PlxyGV-SA shares a sequence identity of 98.6% or 98.7% with all the other viral isolates. The sequence identity between the genomes of PlxyGV-C, -K, -M, and -T is as high as 99.9%. PlxyGV-W and PlxyGV-B, similarly, share sequence identities of 99.5% or 99.4% with PlxyGV-C, -K, -M, -T, -K1, and -Wn. PlxyGV-Wn demonstrates an identical sequence identity of 99.6% with PlxyGV-C, -K, -M, -T, and -K1. PlxyGV-K1has a sequence identity of 99.7% with PlxyGV-C, -K, -M, and -T. The frequency of variation in hrs is much higher than in other regions. The rates of mutation between PlxyGV-W and the other isolates occurring in hrs, other noncoding sequences and ORFs are 53.29 ‒ 96.66, 0.84 ‒ 36.51, and 0.82 ‒ 11.83 per 1,000 bases, respectively (S1 Table). Similar variation frequencies in hrs, other noncoding sequences and ORFs are also observed between genomes of the other viral isolates. Base transitions account for most variations between the genomes of the virus isolates. Base transitions are two to three times of transversions between PlxyGV-W and PlxyGV-B, -Wn, -K1, -C, -K, -M, and -T genomes, and nine times of transversions between PlxyGV-W and PlxyGV-SA genomes.

Identity of genome sequences between nine PlxyGV isolates.
Fig 2

Identity of genome sequences between nine PlxyGV isolates.

The complete nucleotide sequences of nine PlxyGV genomes and concatenated amino acid sequences encoded by the PlxyGV genomes were aligned separately, and the identity levels between the genomes are expressed as percentage. The amino acid sequences of ORF9, 13, 26, 39, 49, 95 and 108 were not included. W, PlxyGV-W; B, PlxyGV-B; C, PlxyGV-C; K, PlxyGV-K; T, PlxyGV-T; M, PlxyGV-M; SA, PlxyGV-SA; K1, PlxyGV-K1.

Except PlxyGV-K1, all the eight additional PlxyGV isolates have 118 putative protein-coding ORFs in common. ORF organization are completely collinear between the genomes of them. PlxyGV-K1 genome was reported having 120 putative protein-coding ORFs. Sequence alignment shows that the ORF38 and ORF39 of PlxyGV-K1 match the upstream and downstream sequences of the ORF38 in the other isolates, respectively. The difference results from a frameshift induced by a single nucleotide deletion in the genome of PlxyGV-K1 relative to the other PlxyGV isolates. Similarly, the sequence of PlxyGV-K1 ORF48 and ORF49 match the downstream and upstream of the ORF49 (p74) of the other PlxyGV isolates. This is also from a single nucleotide insertion/deletion between PlxyGV-K1 and the other isolates. Frameshift variations by single nucleotide changes between PlxyGV-K1 and the other isolates are also found in ORF9, ORF13 (odv-e18), ORF26 (ac23), ORF95 (lef3), and ORF108 resulting in changes in ORF size and predicted amino acid sequences encoded. The ORF9 in PlxyGV-K1 and PlxyGV-SA has extra thirteen codons at the N-terminal relative to its homologs in the other isolates, resulting from a C/T substitution at nt38 upstream of the first ATG of the ORF9 in the other isolates, which creates an new start codon. A single nucleotide missing in the middle of PlxyGV-K1 ORF13 relative to its homologs in the other isolates results in a frameshift after aa48. An A/T substitution in ORF26 creates a stop codon immediate upstream of the second ATG relative to the other PlxyGV isolates. That causes nine codons at the N-terminal missing in PlxyGV-K1 ORF26. A C/A substitution converts the cysteine codon at aa298 in the ORF95 of the other PlxyGV isolates into a stop codon in PlxyGVK1 ORF95, that causes PlxyGV-K1 ORF95 forty aa shorter than its homologs in the other isolates. In addition, PlxyGV-K1 has a cluster of seventeen nucleotides missing in ORF108 relative to the other isolates, after aa137. That causes frameshift and creates a stop codon immediate downstream of the deletion. Whether these differences between PlxyGV-K1 and the other virus isolates result from evolution or sequencing error needs further verification. In addition, there is a cluster of eleven codons inserted within the C-terminal region of PlxyGV-SA ORF50 (p10) relative to the ones in the other PlxyGV isolates.

Apart from the ORFs described above, ORF73 and ORF20 are most variable among the ORFs of PlxyGV isolates (Fig 3). ORF73 is homologous to AC91. Homologs of this gene are found in genomes of all Group I alphabaculoviruses and CpGV in addition to PlxyGV [42]. It is rich in proline and serine/threonine residues. The amino acid sequence from AA69 to AA74 of PlxyGV-SA ORF73 is different from the ones of the other isolates while PlxyGV-B ORF73 misses four amino acids in this region. There is a glutamine residue at AA96 (AA92 for PlxyGV-B) position in ORF73 homologs of PlxyGV-W, -B, -Wn, -T, and -K1. Following the AA96 is a long cluster of repetitive proline residues varying in number among the virus isolates. PlxyGV-K ORF73 miss all the C-terminal sequences after AA102. The C-terminal of PlxyGV-W ORF73 is totally different from the ones of the other isolates due to frame shift as mentioned before. The ORF20 homologs consist of 223–235 amino acids. There are 4–6 repeated “RCPSPR” and 4 “RC/SP/Q/S/ESPR/H” repeats in the middle region. PlxyGV-B and PlxyGV-SA ORF20 have two copies, PlxyGV-W and PlxyGV-Wn have one copy of “RCPSPR” less than the other isolates.

Sequence alignment of ORF73 and ORF20 homologs of nine PlxyGV isolates.
Fig 3

Sequence alignment of ORF73 and ORF20 homologs of nine PlxyGV isolates.

The identity levels of total amino acid sequences of the prospective protein products, except ORF9, 13, 26, 73, 95 and 108, between the viral genomes are similar to the identity levels of nucleotide sequences (Fig 2). The prospective amino acid sequences of ORF1, 3, 4 (granulin), 12 (AC145), 13 (ODV-E18), 17 (AC29), 27, 29 (PIF3), 33, 41 (PIF7), 53 (P24), 81, 91 (AC76), 100 (FP), 102, 110, 114 (Ubi-lig), and 119 are identical in all of the nine viral isolates (Fig 1). The sizes of individual ORFs of nine PlxyGV isolates and the non-synonymous substitutions per base pair (NSSP) of each ORF are listed in Table 5. As shown in Fig 4, the NSSP levels of most ORFs are between 0 and 5 per thousand; thirty six ORFs have NSSP levels over 5 per thousand, most are functional unknown. ORF19 demonstrates the second highest NSSP (26.67 per thousand). ORF61-ORF72 and ORF74-ORF102 regions seem more conservative than other parts. These regions contain seventeen baculovirus core genes [42] and only one PlxyGV specific gene. Organization in these regions are similar among all baculoviruses [43].

Nonsynonymous variations in ORFs of nine PlxyGV isolates.
Fig 4

Nonsynonymous variations in ORFs of nine PlxyGV isolates.

Scatter plots represent ORFs with their levels of nonsynonymous sequence substitution per base pair (NSS/bp ×10 −3) demonstrated. Red, green, light blue, orange, black and blue colors indicate structure protein, transcription factor, DNA replication factor, auxiliary, function unknown and PlxyGV unique genes, respectively. Hollow circles indicate baculovirus core genes. ORF71, which shows 158 NSS per base pair (×10 −3) is not included.

Table 5
ORF size and number of nonsynonymous mutations in nine PlxyGV isolates.
ORFNameWBWnCTKMSAK1Number of mutationsNSSP (x10-3)@
166666666666666666600
2P1083838383838383838328.03
310510510510510510510510510500
424824824824824824824824824800
513113113113113113113113113112.54
6PK27427427427427427427427427411.22
7PIF153653653653653653653653653653.11
817517517517517517517517517559.52
9808080808080809393625
10IE139339339339339339339339339343,39
11AC14618018018018018018018018018011.85
12AC14598989898989898989800
13ODV-E1880808080808080809100
1449K44644644644644644644644644632.24
15EXON021621621621621621621621621634.63
16PIF535135135135135135135135135143.8
17AC2958585858585858585800
1831331331331331331331331331355.32
19100100100100100100100100100826.67
202292232292352352352352352351420.93
21P1032032032032032032032032032022.08
22305305305305305305305305305238.74
2313113113113113113113113113112.54
2433833833833833833833833833810.99
2543143143143143143143143443164.64
26F55355355355355355355355354421.21
2779797979797979797900
2821021021021021021021021021023.17
29PIF318118118118118118118118118100
30ODV-E6668268268268268268268268268273.42
3112912912912912912912912912912.58
32LEF2270270270270270270270270270912.35
3397979797979797979700
3413313313313313313313313313312.51
35MMP40240240240240240240240340243.32
36P1326326326326326326326326326311.27
37PIF236836836836836836836836836898.15
3852852852852852852852852815385.05
376
40AC106/10720620620620620620620620620611.62
41PIF753535353535353535300
42V-UBI11411411411411411411411411412.92
4341441441441441441441441441454.03
44130130130130130130130130130615.38
4539K25225225225225225225225125256.61
46LEF1196969696969696969613.47
47SOD15315315315315315315315315336.54
49PIF06276276276276276276276275173.72
578
50P10135135135135135135135146135614.81
51P4738638638638638638638638638643.45
52AC3820720720720720720720720720711.61
53P2415915915915915915915915915900
54AC1315015015015015015015015015012.22
55LEF125125125125125125125125125111.33
56FGF22122122122122122122122122123.02
57100100100100100100100100100413.33
5853535353535353535316.29
59AC15079797979797979797928.44
60LEF686868686868686868627.75
61DBP26326326326326326326326326322.53
6213613613613613613613613813612.45
63AC10337737737737737737737737737721.77
64AC10296969696969696969626.94
6515215215215215215215215215236.58
66AC10136636636636636636636636636632.73
67P6.95656565656565621T5615.95
68AC145/15066666666666666656615.05
69LEF524724724724724724724724724722.70
7038K34034034034034034034034034043.92
71PIF416116116116116116116116116124.14
72HEL111241124112411241124112411241124112492.67
73AC9115315915515810215815015815875158
74ODV-E2521421421421421421421421421411.56
75AC9315615615615615615615615615624.27
76AC9225025025025025025025025025011.33
7713513513513513513513513513524.94
78LEF443243243243243243243243243243.09
79VP3932032032032032032032032032011.04
80ODV-EC2728728728728728728728728728755.81
8110710710710710710710710710700
8234034034034034034034034034076.86
83696969696969696969314.49
84Pif853353353353353353353353353374.38
85TLP2013913913913913913913913913924.80
86AC8119119119119119119119119119111.75
87GP4128328328328328328328328328344.71
88AC7889898989898989898913.75
89VLF134634634634634634634634634643.85
9017517517517517517517517517547.62
91AC7681818181818181818100
92AC7514514514514514514514514514512.30
93DNAPOL978978978978978978978978979103.41
94AC6665165165165165165165165165163.07
95LEF333733733733733733733733729743.96
96PIF612812812812812812812812812812.60
9719419419419419419419419419435.15
98IAP28128128128128128128128128155.93
99LEF949449449449449449449449449442.70
100FP13813813813813813813813813800
101DNA-LIG52352352352352352352352352331.91
10261616161616161616100
103636366666666666666210.58
104FGF39639639639639639639639639643.37
10521421421421421421421421421456.23
106ALK-EXO37837837837837837837837837843.53
107HEL243643643643643643643643643610.76
10828128128128128128128128113833.56
109LEF883883883883883883883883883862.39
11011411411411411411411411411400
11119219219219219219219219219235.21
112UBI-LIG13713713713713713713713713749.73
113198198198197198197197197198915.15
11410910910910910910910910910900
115VP105431131131131131131131131131133.22
116595959595959595959211.30
11724924924924924924924924924922.68
118EGT42942942942942942942942942964.66
11914214214214214214214214214200
120ME5330830830830830830830830830811.08

@ Variations in PlxyGV-K1 ORF13, 26, 38, 39, 48, 49, 95 and 108 were not count in.

The genes unique to PlxyGV

There are thirteen ORFs unique to PlxyGV, including ORF1, 3, 5, 19, 22, 27, 33, 58, 81, 105, 108, 111 and 119. These ORFs do not have any additional homologs in data bases. ORF1, 3, 27, 33, 81, and 119 have identical predicted amino acid sequences among the nine viral isolates. The transcription start sites (TSS) and the polyadenylation sites (PAS) of these PlxyGV-specific genes were determined by Rapid Amplification of cDNA Ends analysis, using total RNAs purified from P. xylostella larvae infected by PlxyGV-Wn. As shown in Fig 5, ORF27 and ORF19 have three and two TSSs separately. The other ORFs have a single TSS. Notably, the TSS of ORF22 is located at 191 nt downstream of the first ATG and 80 nt upstream of the third ATG. The sequences between the two ATGs are highly variable among ORF22 homologs of the PlxyGV isolates. This suggests that the third ATG is the real start codon of ORF22. Similarly, the TSS of ORF105 were localized to 306 nt downstream of the first ATG and 254 nt upstream of the second ATG, implying that the second ATG is likely the real start codon. A TATA box, and A(A/T)CGT(G/T) and CGTGC motifs are present in regions between 40 nt upstream of TSS and initiation ATG, and a baculovirus late promoter motif A/G/TTAAG is located within 100 nt upstream of initiation ATG of individual ORFs are shown in Fig 5 (and S2 Table). Predicted RNA polymerase II TSS motifs CAGT/CAAT/CATT locating between TSS and the initiation ATG are also listed in S2 Table. None of the TSS identified were associated with the TAAG motifs although there are some present in ORF3 (-13), ORF 5 (-73), ORF19 (-13 and ORF58 (-91, and -25). As shown in Fig 5, the TSS of ORF33 and ORF111 extend to upstream of ORF34 and ORF110 respectively.

Structures of the transcription cassettes of PlxyGV-specific genes.
Fig 5

Structures of the transcription cassettes of PlxyGV-specific genes.

The transcription start sites are in bold and underlined, and transcription direction are indicated by arrows. The major promotor elements, TATA boxes, A(A/T)CGT(G/T), CGTGC, and A/G/TTAAG motifs are present in grey bold. Their positions relative to the initiation ATG of the individual PlxyGV-specific ORFs are shown by the numbers above. The initiation ATG and stop codons are in black bold and underlined. The transcription terminator elements, AATAAA and ATTAAA are in black bold and their positions relative to the stop codons of individual PlxyGV-specific ORFs are indicated by the number underneath.

All the PlxyGV-specific ORFs have single PAS (Fig 5 and S2 Table). The PAS of ORF3 and ORF58 are located downstream of ORF4 and ORF57 respectively. In most cases, there is one or two transcription termination signal elements AATAAA or ATTAAA (ORF27) near the PAS. ORF108 lacks a typical AATAAA or ATTAAA. However there is an AATTAAT located 23 nt upstream of the PAS.

Phylogenetic tree of PlxyGV isolates

An evolutionary tree of the nine virus isolates was constructed using MEGA6 software with the neighbor joining method, based on concatenated amino acid sequences encoded by the thirty eight baculovirus core genes [42], using Hyphantria cunea granulovirus (HycuGV) as an outgroup. HycuGV was shown to be most close to PlxyGV [44]. In the process, PlxyGV-K1 ORF48 and ORF49 were merged into one ORF by filling the single missing base relative to their homologs in the other viral isolates. It can be seen that PlxyGV-C clusters with PlxyGV-K. This cluster is near PlxyGV-M and PlxyGV-T. PlxyGV-W and PlxyGV-B are located in the same cluster. PlxyGV-Wn is more distant from PlxyGV-W and PlxyGV-B than the other isolates except PlxyGV-SA. PlxyGV-K1 is closer to PlxyGV-T, -M, -C and -K than PlxyGV-Wn although they are in the same clade. PlxyGV-SA is relatively distant from the other isolates.

Discussion

In this study, we describe the genome sequencing and analysis of three PlxyGV isolates. PlxyGV-W was first isolated from a diamondback moth larva from cabbage fields in Wuhan city, in early 1980s. PlxyGV-B was originally isolated near Beijing in the early 1980s. The PlxyGV-Wn was isolated in Wuhan, in April, 2018. The sequence data show that the genomes of PlxyGV-W and PlxyGV-B share a sequence identity of 99.8%. And the amino acid sequences encoded by these two viral genomes are almost identical except for variations in ORF20 and ORF73. Surprisingly, although PlxyGV-Wn was isolated from the same area as PlxyGV-W, it shares higher sequence identity with PlxyGV-K1, -T, -C, -K and -M than with PlxyGV-W and -B (Fig 2), and is more closely related to PlxyGV-K1, -T, -C, -K and -M than to PlxyGV-W and -B as demonstrated by the phylogenetic tree (Fig 6). It implies that PlxyGV-Wn and PlxyGV-W originated from different populations, which may have emerged in Wuhan at different time points in history. Located at the junction of the Yantze and Han rivers, Wuhan is a transportation hub that facilitates the introduction of species from different regions. In addition, PlxyGV-W and -B demonstrated higher insecticide activity to diamondback moth larvae than PlxyGV-Wn. How the genomic sequence variations determine the differential insecticidal activity between PlxyGV-Wn and the other two virus isolates will require further investigation. Notably, among the fouty eight ORFs containing non-synonymous variations between PlxyGV-Wn and PlxyGV-W and/or PlxyGV-B are homologs of egt and six per os infectivity factor genes pif0, pif1, pif2, pif5, pif6 and pif8 and odv-e66, an additional possible per os infectivity factor gene. Egt encodes ecdysteroid UDP-glucosyltransferase to block molting and pupation in infected larvae, thereby to prolong the feeding stage of infected larvae [45, 46]. per os infectivity factors are required for infection of insects [47–50].

Neighbor joining phylogenetic tree of PlxyGV isolates.
Fig 6

Neighbor joining phylogenetic tree of PlxyGV isolates.

The phylogeny was inferred using concatenated amino acid sequences of the homologs of thirty eight baculovirus core genes of nine PlxyGV isolates.

Previously, intra isolate genetic diversity was reported in many baculoviruses. Phenotypic changes were observed between genotypes. For instance, twenty-five genotypic variants of a nucleopolyhedrovirus were identified and purified from a single Panolis flammea larva. Four of the genotypic variants were found having significant difference in pathogenicity, speed of killing and yield [2]. Genome sequencing makes it possible to characterize inter and intra isolate diversity of same species. Complete NPs contained in a baculovirus genome was first identified in Mamestra configurata nucleopolyhedrovirus v90/4 [4]. It is considered that presence of a pool of polymorphisms may provide advantage in adapting to a changeable environment. In this study, intra isolate NPs are identified in the genomes of all the three PlxyGV isolates. The NPs occurring in PlxyGV-W almost completely overlap with the ones in PlxyGV-B, although PlxyGV-B has more nucleotide polymorphisms than PlxyGV-W. Notably, the ORFs containing non-synonymous polymorphisms include homologs of three DNA replication factors HEL1, LEF5 and DBP, two late expression factors LEF8 and LEF9, and per os infectivity factor PIF9. The NP profile in PlxyGV-Wn genome is totally different from the ones in the other two isolates. The ORFs with non-synonymous polymorphisms in PlxyGV-Wn genome also include a homolog of per os infectivity factor, PIF5.

Genome sequence comparison of nine PlxyGV isolates reveals high genetic stability of PlxyGV. These PlxyGV isolates are from five areas of four countries, but they have limited variations in genome size and nucleotide sequence. The maximal length difference is only sixty three base pairs, which exists between PlxyGV-W/PlxyGV-SA (100,941 bp) and PlxyGV-T (101,004 bp). The minimum sequence homogeneity is 98.6 percent, existing between PlxyGV-SA and four isolates from the mainland of China and the one from Japan. No gain/loss of prospective protein-coding ORFs identified among the viral isolates. The high genetic stability of PlxyGV ensures the stability and specificity of its control effect on diamondback moth, and is helpful to commercialization of PlxyGV insecticides. In addition, it also facilitates the construction of recombinant PlxyGVs with enhanced insecticidal activity through genetic manipulation, ensuring that the superior properties obtained by engineered viruses are not easily lost or changed.

Previously reported genomes of different geographic isolates of the same baculovirus species usually have variations in gene contents, frequently occurring in bro gene associated regions [4, 51]. PlxyGV lacks bro homologs. Similarly, seven Erinnyis ello granulovirus (ErelGV) field isolates also have common ORF contents and organization, but all of them are isolated in Brazil [7]. Similar to other baculoviruses, NPs are mainly present in hrs and two ORFs containing repetitive sequences in PlxyGV genome. All the ORFs with the highest levels of nonsynonymous mutations have unknown functions. Paralogous genes p10 and ac145/ac150 homologs demonstrate relatively high levels of non-synonymous mutations. Baculoviruses have thirty eight core genes whose homologs are present in all baculovirus genomes sequenced to date [42]. Generally, the PlxyGV core gene homologs contain low levels of non-synonymous variations among the nine viral isolates. Similar phenomenon were also observed in ErelGV isolates [7].

Thirteen ORFs specific to PlxyGV are conserved in all the PlxyGV isolates. The TSS and PAS of the ORFs were identified by RACE analysis. The data suggest that all these PlxyGV-specific ORFs are transcribed during infection. Seven of these PlxyGV unique ORFs have no nonsynonymous variation among all the PlxyGV isolates, implying these genes must play important roles in replication and infection of PlxyGV. Notably, none of the PlxyGV-specific genes were found to start transcription from late promotor motifs. We are not sure whether these results reflected the real situation. If the levels of some transcripts starting from TAAG motifs were very low, they might not be detected.

The PlxyGV isolates analyzed in this study are from five geographically separate areas, the mainland of China, Taiwan, Japan, Malaysia and South Africa. Phylogenetic analysis shows that PlxyGV-SA is distantly related to the other isolates, which may reflect their geographic distance from the other isolates. PlxyGV-M from Malaysia, PlxyGV-C from mainland China and the two isolates from Taiwan, PlxyGV-K and -T are closely related. However, PlxyGV-C is distantly related to other three isolates from the mainland of China. It is likely some isolates migrated from one area to another area recently.

Acknowledgements

We thank Professor Qin, Qi-Lian and Professor Zhang, Huan for their kind support and technical assistance in GV counting. Thank professor George F Rohrmann for critical reviewing the manuscript.

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